Blitzer attributed much of the home price increase for May to seasonal effects. Spring is the hottest time of year for home buying and the added demand usually drives prices higher.

Taking those seasonal factors into account, the 20-city index was flat and the 10-city showed a gain of just 0.1%.

Sixteen metro areas recorded non-seasonally adjusted month-over-month gains in May. The biggest winner was Boston, where prices jumped 2.7%, followed by Minneapolis at 2.6% and Washington at 2.4%. The nation's capital was the only place to record a gain over the past 12 months, up 1.3%.

Three cities declined month-over-month, led by Detroit with a 2.8% drop, Las Vegas, with a 0.9% decline, and Tampa, where prices fell 0.6%. The biggest loser over the past 12 months was Minneapolis, where prices fell 11.7%.

Case and Shiller's takes

The positive aspects of the report has caused Robert Shiller, the Yale economist who, with his colleague, Karl Case, devised the index, to soften -- a little -- what had been an alarmingly pessimistic prediction for the housing market.

He had warned back in February that home prices could drop another 10% to 25% over the next few years.

"The recent data lowers the probability of that scenario a little bit, but I still worry about it," he said.

"There is a big difference between bouncing along a downward drift and bouncing along a rocky bottom," he said. "The fact that prices in a repeat sales index and using a three month moving average actually went up, seems to me, says something important."

Case, however, also mentioned several negative factors having impact on the housing market right now or that will in the near future.